1. Field of the Invention
The present invention relates to an arc plasma generation apparatus suitable for furnace melting, welding, and cutting applications. More particularly, the invention relates to an arc plasma torch equipped with a tapered bore electrode.
2. Description of the Related Art
The use of arc furnaces equipped with arc plasma torches is common for melting and refining applications involving metals and alloys. Furnaces employing arc plasma torches are particularly useful in melting reactive metals because such metals rapidly react or splatter when heated in certain atmospheres.
A typical arc plasma torch employs a cylindrical, straight-bore electrode; a gas-constricting nozzle, spaced away from the electrode; a chamber which surrounds the space between the electrode and the nozzle; and a means for generating a vertical flow of pressurized arc gas which extends back up into the chamber and bore of the electrode and swirls down through the front of the nozzle. This type of design is often referred to as a swirl flow torch. Because of the nozzle's constricting effects, the plasma arc resembles a column.
In the presence of an arc, the pressurized arc gas becomes ionized, thereby forming an arc plasma which is expelled through the constricting nozzle as a swirling, superheated plasma jet. The swirling arc gas also helps to protect the electrode from erosion or contamination because the point on the electrode from which the arc emanates (arc termination point) tends to spin with the arc gas instead of remaining at a singular spot.
An arc plasma torch develops heat by a plasma arc which is drawn between the arc plasma torch electrode and the workpiece (often called the transferred mode). Alternatively, heat may be developed between a torch electrode and a second, external electrode (called non-transferred mode). The transferred mode is usually more efficient because energy transfers directly from the torch to the workpiece, rather than partially dissipating to a separate electrode.
Most advantages offered by plasma arc melting relate to the columnar properties of the arc. Constriction of the plasma arc into a column increases the directional stability of the arc. Thus, the arc is stiffer and is easier to focus in the direction pointed. The constricted arc has high current density and high heat energy concentration in a narrow zone. Because the arc is column-shaped, it also has less sensitivity to differences in arc length and torch stand-off.
The prior art includes designs both for generating arc plasma and for incorporating material for treatment by such plasma. Baird (U.S. Pat. No. 3,194,941) and Camacho (U.S. Pat. No. 3,673,375), both incorporated herein by reference, exemplify two prior art approaches to arc plasma torch design.
Baird (U.S. Pat. No. 3,194,941) is believed to have developed the original swirl flow torch sold by Union Carbide Corporation. Baird instructs that the ratio of the nozzle length (B) to the nozzle inside bore (C) is critical. Recommended values of B/C are between 1.2 and 3.0, with 2.0 being the optimal ratio. According to Baird, values of B/C less than 1.2 cause double arcing. Baird also teaches that much greater values of B/C make arc transfer difficult and reduce the heat efficiency of the arc effluent.
The prior art further includes U.S. Pat. No. 4,718,477 (the '477 patent) issued to Camacho, which is also incorporated herein by reference. It discloses that plasma torch operation in a vacuum results in a significant reduction of the voltage gradient (arc voltage divided by arc length) as compared to operation under atmospheric pressure, which in turn significantly reduces the available output power of the torch for a given arc length.
The '477 patent further states that even though the power level is proportional to the arc length, under vacuum conditions, the voltage gradient may be so low that an increase in arc length provides little increase in power. The '477 patent seeks to overcome the problem of low power levels in the arc by positioning a reduced diameter nozzle just forward of the cylindrical, straight-bore electrode so that the vertical gas flow induced between the electrode and the nozzle generates a back pressure upstream of the nozzle. The effect of this that the portion of the arc upstream of the nozzle is subjected to a relatively higher pressure which in turn increases the voltage gradient. As a result, the overall length of the arc can be increased and greater power levels can be achieved.
It is noted, however, that the increase in arc length is upstream of the nozzle so that the effective arc length outside the torch, that is, between the end of the torch and the pool of metal being heated by the plasma, does not change and remains relatively short. As a result, the "stand-off" length of the torch, that is, the length of the portion of the arc between the molten pool and the torch end, remains relatively short. Consequently, large pieces of metal that are being fed into the furnace for melting may contact the end of the torch and cause shorting and torch damage.
It is well known that maintenance of a long arc length between the torch and the workpiece is desirable because, generally speaking, this provides the arc with greater power. A concomitant benefit of a long arc length is a long stand-off distance between the torch and the workpiece. A long stand-off enables easy feeding of material between the molten pool and the torch body without damaging the torch.
Thus, it is an important aspect of plasma melting to generate an arc which is long enough to enable easy feeding of material between the molten pool and the torch body without damaging the torch while maintaining the desired, relatively high power output of the torch. The present invention provides a plasma torch which has these characteristics.